通过ajax发送数据在Post方法中为空值

var data = { key: 'value' };
var jsonData = JSON.stringify(data);

xhr.setRequestHeader('Content-Type', 'application/json');

const express = require('express');
const bodyParser = require('body-parser');
const app = express();

app.use(bodyParser.json());

xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
        console.log(xhr.responseText);
    }
};

app.use((req, res, next) => {
    res.header('Access-Control-Allow-Origin', '*');
    res.header('Access-Control-Allow-Headers', 'Origin, X-Requested-With, Content-Type, Accept');
    next();
});

var xhr = new XMLHttpRequest();
var url = 'your_api_endpoint';
var data = { key: 'value' };
var jsonData = JSON.stringify(data);

xhr.open('POST', url, true);
xhr.setRequestHeader('Content-Type', 'application/json');

xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
        console.log(xhr.responseText);
    }
};

xhr.send(jsonData);